Three Mile Island Nuclear Generating Station has been a nuclear power plant located on the Susquehanna River, south of Harrisburg, Pennsylvania, since 1974. On March 28, 1979, one of the plant's units, TMI-2, had a partial meltdown, resulting in the biggest accident in U.S. commercial nuclear energy. The incident occurred due to a non-nuclear secondary system failing, which caused a stuck valve to leak radioactive gases. Thousands of residents were evacuated. The incident caused no deaths or physical injuries, but the Nuclear Regulatory Commission admitted to Congress after the incident that it could not confirm how much radiation was released into the atmosphere. Millions have been paid by the plant owners to residents whose children were born with birth defects. At least 2,400 residents filed class-action lawsuits and many have said that the state has covered up the incident's long term impact.